1. Field of the Invention
The present invention relates to a positively chargeable toner for developing electrostatic latent images used for electrophotography, electrostatic recording, electrostatic printing and other purposes.
2. Description of the Prior Art
In copying machines and other equipment based on electrophotography, various dry or wet toners containing a coloring agent, a fixing resin and other substances are used to visualize the electrostatic latent image formed on the photoreceptor having a light-sensitive layer containing an inorganic or organic photoconductive substance.
The chargeability of such toners is the most important factor in electrostatic latent image developing systems. Thus, to appropriately control or stabilize the charge amount of toner, a charge control agent providing a positive or negative charge is often added to the toner.
Examples of charge control agents providing a negative charge for toner in actual application include chromium complex salts, cobalt complex salts, iron complex salts and other salts of azo dyes having a good charge providing property. Examples of charge control agents providing a positive charge for toner in actual application include basic dyes such as nigrosine dyes and triarylmethane dyes. Most of these, however, are relatively densely colored.
For general applicability to color toners of various colors, the charge control agent must be colorless or colored lightly to the extent that the tone of the color toners is not affected. There are a large number of positive charge control agents meeting these requirements, including quaternary ammonium salt compounds (e.g., Japanese Patent Examined Publication No. 54696/1989), pyridinium salt compounds (e.g., Japanese Patent Examined Publication No. 29062/1992), imidazole compounds (e.g., Japanese Patent Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 262555/1989), triazine compounds (e.g., Japanese Patent O.P.I. Publication No. 141072/1988), amine compounds (e.g., Japanese Patent O.P.I. Publication No. 90864/1984) and polyamine resins (e.g., Japanese Patent Examined Publication No. 13284/1978).
Among the toners incorporating conventional positive charge control agents, however, many are unsatisfactory in charge stability in multiple repeated use; for example, the toner charge distribution is uneven due to the poor compatibility or dispersibility of the charge control agent with the toner resin, and the resulting charge is not stably retained due to a lack of environmental stability. Against this background, there is a need of the development of black toners and color toners containing a positive charge control agent improved with respect to such properties.
In recent years, some toners positively or negatively charged by introducing a polar group to the resin itself or by other means have been proposed, but they have various problems to be solved as to chargeability and other properties.
The object of the present invention, developed in view of the above problems involved in prior art methods, is to provide a positively chargeable toner for developing electrostatic images which is versatile for various color toners, including those of the three primaries, and achromatic toners, which is excellent in charge stability to changes in temperature and humidity, i.e., environmental resistance, stability of chargeability over time, i.e., storage stability, and stability of chargeability in multiple repeated use, i.e., durability.